1. The Field of the Invention
The present invention relates generally to systems and devices for use in securing semiconductor elements of solid state devices. More particularly, embodiments of the present invention relate to a mechanical clamping assembly which facilitates uniform application of clamping forces to the semiconductor elements of high voltage solid state devices so as to ensure substantial thermal and electrical communication between the semiconductor elements, and thereby contribute to safe, reliable, and effective operation of the solid state device.
2. The Relevant Technology
Semiconductor materials, and devices that employ them, have been in use for some time. The physical and chemical structure of semiconductors makes them especially well suited for use in a variety of applications. In particular, it is well known that some materials such as silicon and germanium are arranged into regular atomic patterns or structures, often referred to as crystals. One characteristic of such materials however is that they are not particularly effective in conducting electricity, nor are they well suited for use as electrical insulators. Because such materials are not particularly useful for electrical conduction, or for impeding electrical conduction, they are generally referred to as xe2x80x9csemiconductors.xe2x80x9d
It is well known however, that by adding various amounts of certain other atoms to the crystal structure of materials such as silicon and germanium, in a process sometimes referred to as xe2x80x9cdoping,xe2x80x9d these materials can made to assume certain desirable characteristics. In particular, the doping of materials such as silicon and germanium allows manufacturers to produce materials that have particular desired electrical properties. For example, some semiconductor materials can be doped so that they possess substantially improved electrical conductivity in some situations. Conversely, other semiconductor materials can be doped in such a way that they are substantially resistant to conduction of electricity in certain situations. Notwithstanding the desirable electrical characteristics of doped materials however, both doped and undoped materials are referred to generally as semiconductors.
Thus, semiconductors possess a number of desirable properties. The ability of semiconductors to be modified in a variety of ways to facilitate achievement of particular results or effects makes useful in a variety of applications. Further, because the semiconductors comprise solid crystals, they are resistant to rough handling and vibration.
Because the semiconductor material takes a solid form or xe2x80x9cstate,xe2x80x9d electronic parts, components, and devices employing semiconductor materials are often referred to as xe2x80x9csolid statexe2x80x9d devices. Solid state devices are embodied in a virtually endless variety of forms. Examples of common solid state devices include relays, thyristor switch assemblies, transistors, and diodes. Furthermore, there are numerous fields of application for solid state devices. For example, solid state devices are commonly employed in lasers, radar systems, x-ray tubes, and the like.
Solid state devices are constructed in any of a variety of different ways. One common construction method involves stacking a plurality of semiconductor elements. For example, in the case of high voltage solid state devices, a sufficient number of semiconductor devices must be connected together in series to obtain the required voltage rating. Thus, to operate such a solid state device at 10,000 volts, three semiconductor devices rated at 4,500 volts each would be connected in series to obtain an aggregate rating for the solid state device of 13,500 volts.
Typically, the stack of semiconductor devices, or xe2x80x9cstackxe2x80x9d elements, are subjected to large compressive forces, sometimes as high as 10,000 pounds, in order to enhance their operational characteristics. Application and maintenance of the compressive force are important to the overall operation of the solid state device in that the compression facilitates substantial contact between the individual stack elements. This substantial contact, in turn, facilitates a high degree of thermal and electrical communication between the various stack elements. Because some of the stack elements typically comprise heat sinks or the like, a high level of thermal communication between the stack elements facilitates effective removal of heat from the solid state device. In similar fashion, good electrical communication between the stack elements facilitates safe, reliable, and effective operation of the solid state device.
In order to supply the large compressive forces necessary to facilitate effective and reliable operation of the solid state device, a variety of clamps and clamping devices have been devised. As discussed below, however, many known clamping devices suffer from a variety of shortcomings that render them ineffectual and/or cause damage to the stack elements. Such shortcomings are further aggravated when attempts are made to use known devices in high voltage environments.
Some of the problems and shortcomings with known clamping devices relate to the materials used to construct those devices. In general, known clamping devices typically employ upper and lower clamping plates joined together by a number of compression bolts. Typically, the stack is disposed between the upper and lower compression plates and nuts on the compression bolts are then tightened as required to move the upper and lower clamping plates together, and thereby compress the stack elements together.
In high voltage applications, in particular, the voltages involved are so large that the spacing between the stack and the compression bolts is insufficient to prevent arcing between the stack elements and the compression bolts. Accordingly, many known clamping devices that have been developed for use in high voltage environments employ compression bolts comprising fiberglass or other electrically non-conductive material. While the fiberglass material represents an improvement in that it substantially prevents arcing between the stack elements and the compression bolts, it has certain inherent shortcomings.
A significant problem with the fiberglass compression bolts relates to the relative softness of the fiberglass material. Specifically, clamping forces as high as ten thousand pounds are required in some high voltage applications to establish and maintain the contact between the stack elements that is necessary for effective and reliable operation of the solid state device. Thus, the nuts must be tightened to a high degree to produce such clamping forces. As a result of the high clamping forces that they are required to impose, the relatively soft threads of the fiberglass compression bolts are vulnerable to stripping.
A related problem with compression bolts constructed of fiberglass and like materials concerns the difficulty of tightening the nuts on the compression bolts in a symmetric fashion. That is, the nuts must be tightened so that the compression force exerted by the clamping device is distributed evenly across the upper surface of the stack. This arrangement is necessary to ensure consistent and substantial contact between the stack elements and thus, effective and efficient operation of the solid state device. As discussed below, this result is difficult to achieve in practice, and it is generally the case that the nuts on the compression bolts are tightened at least somewhat asymmetrically.
A typical consequence of even slightly asymmetric or unbalanced tightening of the nuts on the compression bolts is that the magnitude of the mechanical stresses and strains between the nut and corresponding compression bolt in the region of relatively greater stack compression is materially higher than the magnitude of the mechanical stresses and strains between the nut and corresponding compression bolt in the region of relatively lesser stack compression. The high stresses and strains that typically result from asymmetric tightening of the nuts on the compression bolts can cause stripping of the compression bolt threads, thereby requiring removal and replacement of the compression bolt.
In addition to the problems inherent in the use of compression bolts made of fiberglass or similar materials, known clamps and clamping devices suffer from other shortcomings as well. One such problem relates to the magnitude of the clamping forces employed in compressing a particular stack of elements. As previously discussed, many known clamping devices employ a plurality of compression bolts or the like which are used to provide the compression required for reliable and effective operation of the solid state device. However, it is difficult, if not impossible, for the assembler of the solid state device to determine with any degree of certainty the magnitude of the clamping force being applied as the nuts on the compression bolts are tightened. Thus, an assembler may inadvertently overtighten or undertighten the nuts on the compression bolts. As discussed below in detail below, undesirable consequences are implicated in either case.
Specifically, where the nuts on compression bolts are overtightened, the resulting compressive force exerted on adjacent regions of the stack may become sufficiently large in magnitude as to cause damage to the stack elements. In many cases, this damage will be imperceptible and may not be fully appreciated until attempts are made to energize and use the solid state device. Alternatively, in the case of undertightening, if the compressive force exerted by the clamping assembly on the stack is of insufficient magnitude, the contact between the stack elements that is required for efficient operation of the solid state device will not be achieved. Thus, it is important that the assembler be able to readily ascertain the magnitude of the clamping force being exerted upon the stack by the clamp or clamping device.
Not only is it difficult, in the context of known clamps and clamping assemblies, to ascertain the magnitude of the compressive force being applied to the stack, but known clamps and clamping assembly typically make no provision for a reliable guide that would insure that the compressive force applied is uniformly distributed across the upper surface of the stack. Rather, the assembler is typically compelled to tighten the nuts on the compression bolts in some type of cross-tightening manner in an attempt to insure uniform application of the compressive force to the stack. Such methods are unreliable, however, and typically result in an imbalance in the compressive force applied to the stack.
In particular, the nuts on some of the compression bolts are secured more tightly than others. As a result, the compressive force applied by the clamping device is not uniformly applied across the top of the stack, and the compressive force is relatively greater in some regions of the stack than in others. Such imbalances in the magnitude of the compressive force applied to the stack can cause crushing or other damage to stack elements in the region of the relatively higher compressive force and, at the same time, may result in insufficient contact between the stack elements in the region of the relatively lower compressive force.
Finally, known clamping devices and systems are generally ill-equipped to compensate for the horizontal misalignment of stack elements that commonly occurs. In particular, due to causes such as variations in the manufacturing processes used to produce the stack elements, it is often the case that stack elements are of varying thickness. That is, some regions of a particular stack element may be thicker than other regions of the same stack element.
Accordingly, when a plurality of stack elements are employed to construct a stack of a solid state device, it often occurs that the upper surface of the stack is not horizontal, or is horizontally misaligned. In particular, one side of the stack is relatively higher than the other side of the stack. Thus, even if the compression bolts are tightened uniformly, a force imbalance on the stack occurs because one side of the stack is higher than the other side of the stack. In particular, the compressive force on the relatively higher side of the stack is of relatively larger magnitude than the compressive force on the relatively lower side of the stack. As discussed elsewhere herein, such force imbalances materially compromise the operation and reliability of the solid state device.
Even in instances where horizontal misalignment of the stack elements could be readily perceived, known clamping devices provide no effective way to compensate for, or remedy, the misalignment. While attempts could be made to overtighten the nuts of some compression bolts and undertighten others, so as to achieve application of a compressive force somewhat more evenly distributed across the top of the misaligned stack, such relative overtightening and undertightening implicates, as described earlier, a variety of undesirable effects with regard to the compression bolts, and the overall integrity and operation of the solid state device.
In view of the foregoing problems and shortcomings with existing clamping devices and assemblies, it would be an advancement in the art to provide a clamping assembly that applies the compressive force evenly across the top of the stack. Additionally, the clamping assembly should facilitate ready ascertainment of the magnitude of the compressive force being applied to the stack. Finally, the clamping assembly should be suitable for use in high voltage environments.
The present invention has been developed in response to the current state of the art, and in particular in response to these and other problems and needs that have not been fully or adequately solved by currently available clamping assemblies. Thus, it is an overall object of embodiments of the present invention to provide a clamping assembly that facilitates relatively uniform distribution of the clamping force across the top of the stack notwithstanding any misalignment of the stack, and that facilitates ready ascertainment of the magnitude of the compressive force being applied. Embodiments of the present invention are especially well suited for use in the context of high voltage solid state devices. However, it will be appreciated that various features and advantages of the present invention may find useful application in other environments as well.
In a preferred embodiment, the clamping assembly includes a clamp frame having upper and lower clamping plates, preferably comprising aluminum or the like, connected by four dielectric rods. The dielectric rods preferably comprise G-10 or FR-4 laminate and are removably pinned at each end to the respective clamping plates so that the clamping plates are separated by a space. The elements of the stack include a plurality of semiconductors, preferably disk type, such as high current thyristors or the like. Heat sinks, preferably comprised substantially of copper and configured for fluid communication with a source of coolant, are interposed between the disk type semiconductors so as to remove heat therefrom.
The stack of semiconductors and heat sinks is disposed between the space defined by the upper and lower clamping plates and is secured so as to prevent lateral motion of the stack elements with respect to each other, and with respect to the clamp frame. Securement of the stack elements in this manner is preferably accomplished by way of a plurality of metal alignment pins removably received in cavities defined in the stack elements and one or both of the clamping plates.
A plurality of spring washers are disposed in a recess defined in the upper clamping plate. A compression cap on top of the upper clamping plate, and secured to the upper clamping plate by a plurality of cap screws, facilitates adjustments to the compressive force exerted by the spring washers. Specifically, adjustments to the cap screws provide relative changes in the deflection imposed upon the spring washers by the compression cap. Such changes in deflection of the spring washers translate to changes in the magnitude of the compressive force exerted by the spring washers and transmitted to the stack.
The clamping assembly further includes a force distribution member, preferably comprising a substantially cone shaped geometry having a substantially circular, flat surface at one end aligned with and in substantial contact with the top of the stack. At the vertex of the cone, a socket is defined in which a pivot ball, preferably comprised of chrome steel, is received, so as to facilitate relative motion between the pivot ball and the force distribution member. A portion of the pivot ball not disposed in the socket is seated within a recess collectively defined by the spring washers. A hole defined in the compression cap communicates with that recess so as to enable ready measurement of the distance from the top portion of the pivot ball received in the recess to the top of the compression cap.
In operation, the cap screws are tightened to the extent necessary to achieve a desired compressive force on the stack. In particular, tightening of the cap screws causes the compression cap to advance so as to exert a force of predetermined magnitude on the ring washers. In response to imposition of that force by the compression cap, the spring washers transmit a proportional compressive force to the pivot ball seated in the recess defined by the spring washers. Because a portion of the pivot ball is received in the socket defined by the force distribution member, the pivot ball acts to transfers the compression force to the force distribution member which, by virtue of its flat circular surface, distributes the compression force substantially uniformly across the top of the stack.
Various features of embodiments of the present invention facilitate ready determination of the magnitude of the compressive force imposed by the clamping assembly on the stack. In particular, the magnitude of that compressive force is a function of the spring constant xe2x80x9ckxe2x80x9d of the spring washers and the distance xe2x80x9cxxe2x80x9d that the spring washers are compressed. Because k is known, or can be readily determined, for particular spring washers and because the distance that the spring washers are compressed can be measured by way of the hole defined in the compression cap, the magnitude of the compressive force exerted by the spring washers can be readily determined. In a preferred embodiment, a mechanical indicator or the like automatically displays the distance from the top of the pivot ball to the top of the compression cap, so that no measurement step is required.
Because the spring washers transmit the compressive force indirectly, rather than directly, to the stack by way of the pivot ball and the force distribution member, asymmetric tightening of the compression bolts, should it occur, will not cause any harm to the stack. Furthermore, because the force distribution member is arranged for rotational movement with respect to the pivot ball, any horizontal misalignment of the stack is readily accommodated by movement of the force distribution member. In this way, the force distribution member remains in substantial contact with the upper surface of the stack, notwithstanding any horizontal misalignment of the stack. As a result, the applied compressive force is, in all cases, uniformly distributed across the upper surface of the stack. Finally, because the rods attaching the upper and lower clamping plates are comprised of a dielectric material, embodiments of the present invention are particularly well suited for use in high voltage applications.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.